Ever since ceramics became popular for use in electrical instrumentation, product design engineers have sought to fasten or bond ceramic parts to other materials such as metal or glass. Because of specialized assembly requirements, frequent issues have arisen regarding adhesion, functionality and longevity. The factors that most commonly challenge specialty adhesives include intense heat and high electrical current.

Specialty adhesives offer solutions to many electrical assembly needs, but what makes one high-temperature adhesive better than another in the minds of design engineers?

Market Survey

Sauereisen Inc. of Pittsburgh, Pa., sponsored an independent survey within a sector of the industrial market to determine what properties and qualities differentiate high-temperature adhesives. The chosen industrial sector, "Manufacturers of Heating Elements," was selected using Standard Industrial Classification (SIC) Codes. Sauereisen targeted this audience based on the propensity of such companies to bond ceramic pieces to diverse materials while electrically insulating the internal components of their finished products.

Industrial Research Services (IRS), a market research firm located in Tarentum, Pa., conducted the survey in January 2006. Julie Pitkavish, project manager of IRS, telephoned product design engineers and general managers at 245 companies located throughout the U.S. In attaining the response rate of 13%, a fairly robust percentage in the field of market research, IRS provided the data of a statistically relevant sampling.

Figure 1. Respondents rated the importance of product attributes when selecting an adhesive.

The Results

For the purposes of the survey, high-temperature applications were defined as those with an operating temperature of at least 500°F (260°C). As shown in Figure 1, respondents considered the most important product attributes of high-temperature adhesives to be:

Physical properties-exhibiting specific qualities such as strength and thermal and electrical characteristics.

Given the broad spectrum of physical properties that could play an important role in adhesive applications, IRS sought clarification on which physical properties were most critical. The most common answers were temperature resistance, thermal conductivity and dielectric strength (see Figure 2).

Where product problems had occurred, the most prominent challenges were identified as waste and failure to meet electrical and thermal requirements. In addition, the most frequently cited product improvement opportunities included greater temperature resistance and faster curing rates.

Drawing Conclusions

Given that information, what type of adhesive is best? The answer depends on the operating conditions of the instrumentation. Both organic and inorganic technologies offer distinct advantages. Either chemistry can be formulated with fillers to impart relatively high levels of thermal conductivity and electrical insulation. Organics, including epoxies, phenolics, polysulfides and urethanes, usually offer a good bond to smooth substrates like plastic and stainless steel. In addition, organic adhesives may exhibit lower permeability and higher mechanical strengths than inorganics, at least to the point of their maximum service temperature.

Inorganic adhesives, such as those with a silicate, phosphate or calcium aluminate binder system, show superiority when subjected to high temperatures. These types of adhesives, or cements, can retain their strengths through temperatures of 3000°F (1648°C). In most cases, this superiority will be accompanied by greater resistance to thermal shock, since an inorganic's lower coefficient of thermal expansion (2.5-15.0 x 10-6/°F) poses less internal stress than that of the typical epoxy bonding agent (20-40 x 10-6/°F).

The same adhesive attributes that are attractive to assemblers of heating elements have been proven to meet the needs for resistor, sensor, igniter and halogen lamp manufacturers. Technology advances relative to temperature limits and handling properties should be a priority to manufacturers or specifiers of adhesives for electrical applications.